Split transaction protocol for a bus system

ABSTRACT

A method of and apparatus for communicating between a host and an agent. The method includes the step of performing a first transaction between a host controller and a hub. The hub is operable to perform a single transaction with an agent based on the first transaction. The method then includes the step of performing a second transaction between the host controller and the hub. The second transaction is based on the single transaction.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/719,640,filed 8 Mar. 2010, which is a continuation of application Ser. No.10/939,365, filed 14 Sep. 2004, U.S. Pat. No. 7,675,871, which is acontinuation of application Ser. No. 09/362,991, filed 27 Jul. 1999,U.S. Pat. No. 6,813,251, the contents of which are incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention pertains to the field of data communication in adigital system. More particularly, the present invention relates tomethods or protocols fOr transferring, information on a bus.

BACKGROUND OF THE INVENTION

A computer or similar device typically has a bus that connects devicesto the computing system. Due to a variety of advances (e.g., computingpower) in computing systems and devices, the amount of data that can beexchanged between a computing system and its attached devices hasincreased, requiring a concomitant increase in the bandwidth of the bus.Part of this increased demand for bandwidth has come from multimediaapplications that require data to be transferred at regular timeintervals (isochronous) either from the device to the computing system(in) via the bus, or in the opposite direction (out). Examples ofdevices requiring significant bandwidth, include cameras, compact discplayers, speakers, microphones, video display devices, scanners, andjoysticks and mice, among other devices. The bandwidth available on abus architecture is partly determined by three factors: transmissionmedium, topology, and the protocol used to control access to the medium.The protocol and topology partly determine the nature of therelationship between a device and a computing system. One possiblerelationship is a master-slave relationship. In a master-slaverelationship the computing system initiates typically all datatransactions between the computing system and a device; i.e., the deviceonly responds to requests from the computing system but never initiatesa transaction. A benefit of the master-slave relationship is a busarchitecture having relatively low cost and simplicity. The UniversalSerial Bus (USB) Specification Revision 1.1, Sep. 23, 1998, is anexample of a popular bus architecture having a master-slave relationshipbetween elements connected by the bus. Unfortunately, many of today'sdevices and computing systems have bandwidth requirements (or datarates) that cannot be supported by existing master-slave bus standards,such as the USB Standard.

Even though USB does not support relatively high data rates, it has arelatively large base of users, and supports two data rates: 12 Mb/s(“full-speed”) and 1.5 Mb/s (“low-speed”). USB allows multiple datarates on a bus because some devices do not require high data rates andcan achieve significant cost savings by taking advantage of relativelylow-cost, low data-rate drivers and cabling.

However, the USB protocol that allows the computing system tocommunicate at low-speed with low data rate devices and alternatively atfull-speed with high data rate devices (speed-shifting) results in theamount of data actually transmitted over the bus (effective throughput)being less than that achievable by limiting the bus to full-speedtransactions, hi other words, speed shifting results in less bandwidthbeing available for higher speed (e.g., full-speed) devices, especiallywhen there is a relatively large number of low-speed devices attached tothe computing system. The effect of speed shifting on throughput isexacerbated where the ratio of high data rate to low data rate isrelatively large.

Another possible bus protocol would require the host to (1) transmit ata high data rate a packet to a hub, (2) wait for the hub to forward atthe low data rate the packet to the agent, (3) wait for the agent torespond at the low data rate to the hub, and (4) receive from the hub ata high data rate the agent's response to the packet. When the ratio ofthe high data rate to the low data rate is relatively large, this busprotocol may also result in a low effective throughput or bandwidthbecause of the need to wait for the hub to forward the packet at the lowdata rate and for the agent to respond at the low data rate.

Another popular bus technology is defined by “Firewire” or Institute ofElectrical and Electronics Engineers (IEEE) Standard 1394 for a HighPerformance Serial Bus, 1995. IEEE 1394 supports multiple data rates, upto 400 Mb/s. While the aggregate bandwidth is substantially higher thanUSB, TREE 1394 employs wasteful speed shifting and is a relativelycostly technology.

The performance of a bus can be significantly affected byspeed-shilling, waiting for a hub to perform transactions at a lowerdata rate than a host data rate, and the ratio of the host's data rateto the agent's data rate. Thus, it is desirable to have a protocol thatallows communication at the higher data rates required by today'sbandwidth intensive systems while allowing backward compatibility withpre-existing solutions, such as USB, and without having to pay thepenalties imposed by speed-shifting and the other disadvantages of theprior art.

One issue faced by high data rate systems communicating with low datarate devices has been described above. Another issue faced by computingsystems arises from the multiplicity of bus protocols (or standards)that are available. Typically, a device manufactured to operate inaccordance with a bus protocol will not operate in accordance with adifferent bus protocol. It may be wasteful to require a user to ownlargely duplicate devices simply because of differences in the protocol.Where there is a large base of devices being used that have asignificant economic life, it may be desirable to allow such devices tobe used with a computing system that has a protocol that providesbackward compatibility to the protocol of the legacy devices.

SUMMARY OF THE INVENTION

According to an embodiment of the invention a method for communicatingdata between a host and an agent is described. The method includes thestep of performing a first transaction between a host controller and ahub. The hub is operable to perform a single transaction with an agentbased on the first transaction. The method then includes the step ofperforming a second transaction between the host controller and the hub.The second transaction is based on the single transaction.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings in which likereferences denote similar elements, and in which:

FIG. 1 a illustrates a block diagram of a digital system using aprotocol in accordance with the present invention;

FIGS. 1 b, 1 c & 1 d each illustrates a process showing a method inaccordance with this invention for communicating between a mostcontroller and a hub;

FIG. 1 e illustrates a hub in accordance with the present invention;

FIGS. 2 a & 2 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing a transfer in accordance with thisinvention;

FIGS. 3 a & 3 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention;

FIGS. 4 a & 4 b illustrate state machine diagrams for a host controllerand a huh, respectively, performing another transfer in accordance withthis invention;

FIGS. 5 a & 5 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention;

FIGS. 6 a & 6 b illustrate state machine diagrams for a host controllerand a huh, respectively, performing another transfer in accordance withthis invention;

FIGS. 7 a & 7 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention;

FIGS. 8 a & 8 b illustrate best case and worst case frames for datatransfers in accordance with the present invention; and

FIG. 9 illustrate timing diagrams for host controller-hub transactionsand hub-agent transactions.

DETAILED DESCRIPTION

A method and apparatus for communicating between a host and a peripheral(agent) is described, where the agent communicates at a different speedand/or protocol than the host. In the following description, forpurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be evident, however, to one skilled in the art that the presentinvention may be practiced in a variety of bus systems, especiallyserial buses, without these specific details. In other instances wellknown operations, steps, functions and devices are not shown in order toavoid obscuring the invention.

Parts of the description will be presented using terminology commonlyemployed by those skilled in the art to convey the substance of theirwork to others skilled in the art, such as device or controller drivers,bus or host controllers, hubs, bus agents or agents, and so forth. Also,parts of the description will also be presented in terms of operationsperformed through the execution of programming instructions orinitiating the functionality of some electrical component(s) orcircuitry, using terms such as, performing, sending, processing,packaging, scheduling, transmitting, configuring, and so on. As wellunderstood by those skilled in the art, these operations take the formof electrical or magnetic or optical signals capable of being stored,transferred, combined, and otherwise manipulated through electricalcomponents.

Various operations will be described as multiple discrete stepsperformed in turn in a manner that is most helpful in understanding thepresent invention. However, the order of description should not beconstrued as to imply that these operations are necessarily performed inthe order that they are presented, or even order dependent. Lastly,repeated usage of the phrases “in one embodiment,” “in an embodiment,”“an alternative embodiment,” or “an alternate embodiment” does notnecessarily refer to the same embodiment, although it may.

FIG. 1 a illustrates a block diagram of a bus using a protocol inaccordance with the present invention. Bus 100 includes a system 102having a host controller 110 which is coupled to hub 120 which is inturn coupled to agent 130. Host controller 110 has an associated devicedriver 105 that executes on system 102. Examples of agents includecameras, compact disc players, speakers, microphones, video displaydevices, scanners, and joysticks and mice, among other devices, System102 can include any digital system capable of digital communication,especially laptop computers, desktop computers, servers, set-top boxes,entertainment systems, and game machines. Consequently, this inventioncan be practiced with a variety of digital devices using digitalcommunication.

Two arrows 101 a and 101 b are drawn in FIG. 1 a to provide a frame ofreference as to the direction of communication among the host, hub andagent. The direction from the agent to the hub and on to the host isreferred to as the upstream direction or upstream (in). The directionfrom the host to the hub and on to the agent is referred to as thedownstream direction or downstream (out).

Host controller driver 105 facilitates communications or transactionsbetween along bus 100 (e.g., on behalf of an application executing onsystem 102) by processing packets of information destined for agent 130and scheduling the packets for transmission by controller 110. Hostcontroller 110 sends data to and receives data from agent 130 data viahub 120. Agent 130 communicates at a different or lower data rate (agentdata rate) than the data rate of host controller 110 (host data rate).While only one agent is shown coupled to hub 120, it should be apparentthat additional agents (not shown) can be attached to hub 120 or toother hubs (not shown). These additional agents may communicate at thehost data rate or the agent data rate. Furthermore, while agent 130 isshown coupled to hub 120, it may be coupled to hub 120 through at leastone conventional repeater type hub that operates at the agent data rate.A conventional repeater type hub repeats signals it receives on itsupstream side on its downstream ports, and vice versa. The conventionalrepeater type hub may in turn have one or more agents 130 attached toit.

Host controller 110 and agent 130 have a master-slave relationship whichmeans that the host initiates typically all data transactions betweenthe host and an agent; i.e., the agent only responds to requests fromthe host but never initiates a transaction. Hub 120 hasstore-and-forward buffers not shown) that allow hub 120 to temporarilystore downstream information received from host controller 110 anddestined for agent 130, and to temporarily store upstream informationreceived from agent 130 and destined for host controller 110.

Since agent 130 and host controller 110 communicate at different datarates, it is desirable to enhance the effective throughput on the bus byproviding a protocol that would allow host controller 110 to both (1)communicate at its higher data rate and (2) not have to wait forresponses from agent 130 before engaging in another transaction. Theprotocol of the present invention allows host controller 110 to takeadvantage of the store-and-forward characteristic of hub 120 to allowthe host controller 110 to communicate at its higher data rate and toengage in another transaction instead of waiting for a response fromagent 130, if a response is required. The protocol of the presentinvention also provides robustness and reliability to transactionsperformed between controller 110 and hub 120. Additionally, the hostcontroller and/or hub of the present invention allow increased effectivethroughput on the bus and provide increased responsiveness to agents(not shown) that communicate at the host data rate and that are attachedto hub 120 or other hubs.

FIG. 1 b illustrates a process 150 showing a method in accordance withthis invention for communicating with an agent having a lower (ordifferent) data rate, than the data rate of a host controller. The agentmay also have a different protocol than the host controller. Process 150can be used to effect a variety of information transfers between hostcontroller 110 and agent 130. For ease of understanding process 150 willonly be described here with regards to a bulk out transfer. However,process 150 can be used with other information transfers describedherein, below. In a bulk out transfer data is transferred from hostcontroller 110 to agent 130 via hub 120. The bulk out transfer isdefined according to an embodiment of this invention as an asynchronoustransfer type. However, it should not be concluded from this definitionthat any bulk and/or out transfer need be asynchronous.

At step 152 in process 150 a start split transaction is performed. Thestart split transaction communicates downstream information from hostcontroller 110 to hub 120. Some of the downstream informationcommunicated to hub 120 is temporarily buffered in hub 120. The buffersin hub 120 largely behave in a first-in-first-out (FIFO) manner, and aredescribed in greater detail below. Some time after the downstreaminformation is buffered, hub 120 performs a hub-agent transaction (notshown) with agent 130 based on some of the buffered downstreaminformation. The relative timing of the hub-agent transaction need notbe described herein because one of ordinary skill in the art wouldrecognize that this is an application or implementation detail for whichthere are many possibilities. The hub-agent transaction may result inupstream information being buffered in hub 120. Some time after thedownstream information is buffered, at step 156, a complete splittransaction is performed. The complete split transaction communicatesbuffered upstream information from hub 120 to host controller 110. Therelative timing of the complete split transaction need not be describedherein because one of ordinary skill in the art would recognize thatthis is an application or implementation detail for which there are manypossibilities.

A benefit of the split transaction protocol is that it allows controller110 to initiate communication (start-split transaction) with agent 130,engage in another function, or engage in another communication withanother agent (low data rate or high data rate agent) at step 154, andthen return to complete the communication that was initiated earlierwith the low data rate agent. By communicating using split-transactions,controller 110 communicates at high data rates without speed-shiftingand does not sit idle while waiting for hub 120 to communicate withagent 130. The time that would have been spent being idle can be used tocommunicate with another agent. In an alternative embodiment inaccordance with the present invention, controller 110 may engage inspeed-shifting with some agents while engaging in split-transactioncommunication with other agents.

The start split and the complete split transactions (split transactions)described above may be used to implement a variety of transfer types(e.g., read or write) for communicating data between controller 110 andagent 130. In an embodiment of this invention four transfer types (ortransfer requests) are defined: bulk out/in, control out/in, interrupt,isochronous. It should be apparent to one of ordinary skill in the artthat the spirit and scope of this invention includes other embodimentswith fewer, more or different transfer types. Each of the transfer typesprovides different levels of robustness, reliability, synchronization,asynchronous operation, error detection and correction of thecommunication flow, and other characteristics that should be apparent toone of ordinary skill in the art. For example, bulk out/in provideslarge asynchronous data transfers from controller 110 to agent 130 or inthe opposite direction. Control out/in also provides asynchronous datatransfer from controller 110 to agent 130 or in the opposite direction,but the data is typically control information used to control theoperation of elements (e.g., a tape drive) in agent 130 or system 100.Interrupt provides a periodic data transfer from controller 110 to agent130 or in the opposite direction. If the transfer is not successful,controller 110 may try again in an embodiment in accordance with thisinvention. Isochronous transfer provides a data transfer once everypredetermined time interval. According to an embodiment of the presentinvention, the transfer may occur at any time during the time interval.If the transfer is not successful, controller 110 will not repeat thetransfer. In an alternative embodiment in accordance with the presentinvention, the isochronous transfer may provide for repeat transfers.

The split transactions may include a number of phases depending on thetransfer type being implemented. Each of the split transactions may haveup to three phases: token, data, and handshake. However, depending onthe transfer being performed, some transactions may have fewer phases.In an embodiment of the present invention, bulk and control can use thesame phases in each of their respective split transactions. The phasesfor each of the transfer types described above are shown in Table 1,below. Presence of an “X” in a cell of the table indicates that thesplit transaction for the transfer type has the phase indicated at thetop of the column in which the cell resides. While in this embodimentthe token and data phases are separate for each of the transfer types,in alternative embodiments the token and data phases may be combined. Itshould be apparent that in alternative embodiments transfer types mayhave fewer, more, or even different phases than those shown in Table 1without departing from the scope and spirit of the present invention.

TABLE 1 Start-Split Complete-Split Transaction Transaction TransferHand- Hand- Type Token Data shake Token Data shake Bulk-Control Out X XX X X Bulk-Control In X X X X X Interrupt Out X X X X Interrupt In X X XIsochronous Out X X Isochronous In X X X

FIG. 1 c illustrates in greater detail a process 160 showing a startsplit transaction for a bulk out transfer in accordance with anembodiment of this invention. At step 162 a token packet including hubidentification information, agent and endpoint identificationinformation, transfer type, indicator for specifying direction oftransfer (in or out), and data rate identification is sent from hostcontroller 110 to hub 120. Hub identification information, and agent andendpoint identification information, and direction are together commonlyreferred to here as transaction addressing information. The agentidentification information identifies the particular agent with whichthe host is attempting to communicate. The endpoint identificationinfo/nation identifies a particular portion in the agent with which thehost is attempting to communicate. Examples of endpoints include: leftspeaker and right speaker of a speaker hub, or speaker and microphone oftelephone handset. The transfer type in the transaction addressinginformation is not limited to the types described herein (e.g., bulkout, interrupt, isochronous, control), but can include other types knownin the art without departing from the spirit and scope of thisinvention. Data rate identification specifies the data rate with whichthe hub-agent transaction described in connection with process 150 abovewill be performed. For an embodiment in which the hub-agent transactionis performed in accordance with the USB standard, data rateidentification will specify either 12 Mb/s (full-speed) or 1.5 Mb/s(low-speed). At step 164, a data packet is sent from host controller 110to hub 120. At step 166, a first acknowledgement is received by hostcontroller 110 from hub 120, if the data packet was decoded properly byhub 120. The first acknowledgement indicates whether the data wasdecoded properly by hub 120 or whether hub 120 wants to be tried later(e.g., hub 120 had full buffers and was not able to accept the data).

FIG. 1 d illustrates in greater detail a process 170 showing a completesplit transaction for a bulk out transfer in accordance with anembodiment of this invention. At step 172, a second token packetincluding transaction addressing information is sent from the host tothe hub. At step 174, a second acknowledgement is received by hostcontroller 110 from hub 120, where the second acknowledgement can either(1) include handshake information received by hub 120 from agent 130during the hub-agent transaction described above in connection with FIG.1 b or (2) indicate that hub 120 does not yet have information based onthe hub-agent transaction to forward to host controller 110 (e.g., thehub-agent transaction has not yet been completed). The handshakeinformation indicates whether (1) agent 130 properly received dataduring the hub-agent transaction (ACK), (2) agent 130 indicated that itis not able to operate normally (STALL), or (3) agent 130 indicated thatit wanted to be tried later (NAK). While the first and secondacknowledgements and the handshake information have been described asspecifying certain indicators, it should be apparent to one of ordinaryskill in the art that these acknowledgements and handshakes and otherones described herein may represent other indications. Additionally,acknowledgements and handshakes different from or additional to the onesdescribed herein may be added in an alternative embodiment withoutdeparting from the spirit and scope of the invention.

While the above description has generally been presented in the contextof agent 130 and hub 120 communicating at a lower data rate than thedata rate between hub 120 and host controller 110, those skilled in theart will appreciate that the present invention may be practiced tobridge a lower data rate to a higher data rate instead, or even equaldata rates but different protocols.

While in FIG. 1 only one hub was shown in between the agent and the hostthere can be multiple hubs between any particular agent and the host.While only six transfer types have been described, those skilled in theart will appreciate that other types can be used without departing fromthe scope or spirit of this invention.

Each of FIGS. 2 a, 2 b, 3 a, 3 b, 4 a, 4 b, 5 a, 5 b, 6 a, 6 b, 7 a & 7b illustrates a state machine diagram for performing a transfer using ahost controller and a hub in accordance with this invention. Figureswith an “a” as a suffix show the state machine diagram for a hostcontroller; the state machine may be performed on host controller 110described above in connection with FIG. 1 a. Figures with a “b” as asuffix show the state machine diagram for a hub; the state machine maybe performed on hub 120 described above in connection with FIG. 1 a. Thestate machines illustrated in these figures show processes havingmultiple steps. It should be apparent that some of the steps may bedivided into more numerous steps or combined into fewer steps withoutdeparting from the scope and spirit of this invention. The statemachines are not described in great detail because their operationshould be apparent to one of ordinary skill in the art. For ease ofunderstanding, the processes performed by the state machines are notdescribed separately. Since the processes performed by the statemachines operate in tandem (i.e., each process has steps whose executiondepends on the occurrence of events or steps in the other process), thedescription of the processes are interleaved to facilitateunderstanding.

FIGS. 2 a & 2 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing a transfer in accordance with thisinvention, specifically a split out bulk/control transfer. Process 200and process 260 show the state machine for a host controller and a hub,respectively. Process 200 includes start split transaction, having atoken phase (XOUT) and a data phase (DATAx), which may be repeated up tothree times by the host controller when timeouts occur during the phasesof a transaction attempt between the host controller and the hub. Inresponse to the start split transaction, process 260 will eitherpropagate through states that will accept the data (ACK), respond to ahost controller retry after a communication failure of a hub handshaketo the host controller (ACK, don't accept data), request a hostcontroller retry due to lack of space to hold the start transactioninformation (NAK), or timeout up to three times. Process 200 shows thehost controller response to a complete split transaction (KIN) when thetransaction between the hub and agent was successfully processed(Advance), resulted in a NAK from the agent (NAK), received anindication that the agent was not able to function normally (STALL), wasnot yet completed by the hub or agent (NYET), or the XIN or its responsehad some communication failure and resulted in up to three timeoutsbetween the host controller and hub. In response to the complete splittransaction, process 260 will either indicate that the transactionbetween the hub and the agent has not finished (NYET) or will provide anindication (ACK, NAK, STALL) of what transpired during the transactionbetween the hub and the agent.

FIGS. 3 a & 3 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention, specifically a split out interrupt transfer. Process 300and process 360 shows the state machine for a host controller and a hub,respectively. Process 300 includes a start split transaction, having atoken phase (XOUT) and a data phase (DATAx), which is not repeated bythe host controller because according to an embodiment the interrupt outtransfer is time sensitive and need not be repeated if it is notsuccessful on the first try. In response to the start split transaction,process 360 will either accept the data (ACK), or do nothing. Process300 shows the host response to a complete split transaction (XIN) whenthe transaction between the hub and agent was successfully processed(Advance), received an indication that the agent was not able tofunction normally (STALL), was not yet completed by the hub or agent(NYET), or the XIN or its response had some communication failure andresulted in up to three timeouts between the host controller and hub. Inresponse to the complete split transaction, process 360 will eitherindicate that the transaction between the hub and the agent has notfinished (NYET) or will provide an indication (ACK, NAK, STALL, NYET) ofwhat transpired during the transaction between the hub and the agent.

FIGS. 4 a & 4 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention, specifically a split out isochronous transfer. Process400 and process 460 shows the state machine for a host controller and ahub, respectively. Process 400 includes a start split transaction,having, a token phase (XOUT) and a data phase, neither of which isrepeated. Process 400 does not include a complete split transactionaccording to an embodiment. Process 460 allows the hub to agenttransaction data to be subdivided into multiple sections to minimizebuffering required in the hub; the host controller can send the nextsection of data just before the hub needs to send it to the agent. Inthis case each split start transaction is marked with ALL, BEGIN, MEDDLEor END so that the hub can detect when a communication failure hascaused it to not receive a data section in the correct sequence. Inresponse to the start split transaction, process 460 will accumulate adata payload having one data section (ALL), two data sections (BEGIN,END), or three or more data sections (BEGIN, MIDDLE . . . MIDDLE, END).

FIGS. 5 a & 5 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention, specifically a split in bulk/control transfer. Process500 and process 560 shows the state machine for a host controller and ahub, respectively. Process 500 includes start split transaction, havinga token phase (XOUT), which is repeated up to three times by the hostcontroller when timeouts occur during transaction attempts between thehost controller and the hub. In response to the start split transaction,process 560 will either acknowledge and accept the transaction (ACK,accept xact), respond to a host controller retry after a communicationfailure of a hub handshake to the host controller (ACK, ignore xact), orrequest a host controller retry due to a lack of space to hold the starttransaction information. Process 500 shows the host controller responseto a complete split transaction (XIN) when the transaction between thehub and agent was successfully processed (Advance), received anindication that the endpoint is unable to function normally (STALL), wasnot yet completed by the hub or agent (NYET), ignores data received fromthe hub because it is corrupted, or the XIN or its response had somecommunication failure and resulted in up to three timeouts between thehost controller and hub. In response to the complete split transaction,process 560 will either indicate that the transaction between the huband the agent has not finished (NYET) or will provide an indication(NAK, STALL) of what transpired during the transaction between the huband the endpoint, or send the data received from the agent by the nub tothe host, controller.

FIGS. 6 a & 6 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention, specifically a split in interrupt transfer. Process 600and process 660 shows the state machine for a host controller and a hub,respectively. Process 600 includes a start split transaction, having atoken phase (XOUT). In response to the start split transaction, process660 will accept the transaction (accept xact). Process 600 shows thehost response to a complete split transaction (XIN) when the transactionbetween the hub and agent was successfully processed (Advance), receivedan indication that the endpoint is not able to function normally(STALL), was not yet completed by the hub or agent (NYET), received aNAK from the agent (NAK), retries token phase if data received is anagent retry of the previous transaction request (ignore data), or theXIN or its response had some communication failure and resulted in up tothree timeouts before the host controller gives up communicating withthe agent. In response to the complete split transaction, process 660will indicate that a timeout occurred when the hub was communicatingwith the agent (NYET), or the hub didn't received the start transactionfor this request and had no corresponding response information (STALL)or provide an indication (NAK, STALL) of what transpired during thetransaction between the hub and the agent, or send the data to the hostcontroller.

FIGS. 7 a & 7 b illustrate state machine diagrams for a host controllerand a hub, respectively, performing another transfer in accordance withthis invention, specifically a split in isochronous transfer. Process700 and process 760 shows the state machine for a host controller and ahub, respectively. Process 700 includes a start split transaction,having a token phase (XOUT). In response to the start split transaction,process 760 will accept the transaction (accept xact). Process 700 showsthe host response to a complete split transaction (XIN) when thetransaction between the hub and agent was successfully processed and allthe data has been returned (TAadvance) or there is more data to return(DAadvance) or an error occurred (record error) due to a communicationfailure between the host and hub (timeout or bad cyclic redundancycheck) or the agent (NAK) or a hub problem with the agent (NYET). Inresponse to the complete split transaction (XIN), process 660 willindicate that data received from the agent had a bad cyclic redundancycheck (NAK) or the agent didn't respond (NYET) or the hub had noinformation about this complete-split, or send the data to the hostcontroller either indicating all the data has been returned (DATA0) ormore data is to be returned (MDATA).

It is useful at this point to summarize the description of the aboveprotocol before describing the remaining apparatus and methods of thepresent invention. The protocol described above allows a host controllerto transfer data to or receive data from an agent via a hub. Theprotocol allows the host controller to engage in a first transaction(start split transaction) in which a transfer request is communicated tothe hub. After the host controller performs the first transaction, itmay engage in an intermediate transaction with the same hub or anotherhub without waiting, for the hub and agent to perform the transferrequest (i.e., engage in the transfer of data to or from the agent). Theintermediate transaction may include a transfer request for the agent,another agent on the same hub as the agent, or another agent on yetanother hub. After the hub engages in the transfer of data to or fromthe agent, the host controller performs a complete split transaction (orsecond transaction) to get the result (e.g., data or handshake sent fromthe agent to the hub) of the transfer performed between the hub and theagent. By allowing the host controller the capacity to engage in anintermediate transaction, instead of waiting for the hub to perform thetransfer request (or third transaction) with the agent, the effectivethroughput of a bus using a protocol in accordance with this inventioncan be significantly greater than buses which involve speed-shifting orwhich require the host controller to wait for the hub to perform thetransfer with the agent before initiating another transfer.

While the above protocol defines the sequence of transactions involvedin communicating data across a bus, it does not explicitly describe thetiming for the transactions or transfers which will result in data beingsent to or received from an agent. However, timing of transfers foragents is important because agents typically require data to be sent toor received from the host controller on a periodic (e.g., isochronous orinterrupt) or asynchronous (e.g., bulk or control) basis. Additionally,while the above protocol allows a host controller to perform anintermediate transaction (or even multiple intermediate transactions)between the start split and the complete split transaction, the aboveprotocol does not explicitly describe how transfer requests are storedin the hub and how the hub and agent perform transfer requests withoutrequiring the host controller to wait for the transfer request to beperformed before engaging in an intermediate transaction. The issues notaddressed by the above description of the protocol, namely the timing oftransfer requests and the processing (i.e., buffering and performance)of transfer requests by a hub, are addressed by the followingdescriptions of methods and apparatus in accordance with the presentinvention. The present invention includes a method and apparatus forscheduling transfers of data to and from an agent and a method andapparatus for processing transfer requests at a hub. The method andapparatus for scheduling transfers of data is described first and thenthe method and apparatus for buffering and performing transfer requestsis described second.

Referring again to FIG. 1, the process of scheduling transfers of datafirst begins when system 102 performs configuration for agents attachedto the bus of the system. Configuration can occur upon systeminitialization or boot-up or when an agent is attached to the bus afterinitialization. The scheduling of data transfers depends on the transfertype associated with an agent (or an endpoint of an agent) and theamount of data involved in the transfer. The mariner in which agentshaving associated periodic transfers, such as isochronous and interrupt,are handled is described first, below, and then the manner in whichagents having associated asynchronous transfers, such as bulk andcontrol, are handled is described next.

During the process of configuration, each endpoint of each agent informsthe system of the endpoint's associated maximum data payload size andtransfer type (e.g., isochronous in/out, interrupt, bulk, control). Themanner in and device by which each agent informs the system is wellunderstood by those of ordinary skill in the art and need not bedescribed here. The maximum data payload size is the largest amount ofdata that a transfer to or from an agent will entail. The system relaysthe data payload size and the transfer type to the host controller. Themanner in which the system relays the size and transfer type to the hostcontroller is well understood by those of ordinary skill in the art andneed not be described here. The host controller uses the aforementionedtwo pieces of information associated with each endpoint to generate abudget list for the periodic transfers of the endpoints. In alternateembodiment, a software driver such as the host controller driver or evena hardware driver can generate the budget list and perform thescheduling operations described below. The budget list gives theearliest time that a transfer (sending or receiving a data packet) mayoccur as well as the latest time that a result associated with thetransfer may be available. The earliest time that a transfer can occurdepends on the amount of time taken up by each of the previous transfersassuming that the previous transfers happened under the best ofcircumstances (defined below). The latest time that a result associatedwith a transfer may be available depends upon the amount of time takenup by each of the previous transfers assuming that previous transfershappened under the worst circumstances (defined below) and the timerequired for the transfer under the worst circumstances. The earliesttime that a transfer can occur is important because the host controllerneeds to have the transfer request buffered at the hub before that timeso that as soon as the hub finishes the transfer request that is aheadof the buffered request the hub can turn its attention to the bufferedtransfer request. The latest time that a result associated with thetransfer may be available is important because after the latest time itis substantially certain that the result will be available for the hostcontroller to retrieve. If the host controller were to try to retrievethe result from the hub before the latest time, meaning that the resultis not available yet, a less efficient protocol employing multipleretrievals would be required.

Transfers happen under the best of circumstances when each transferinvolves the maximum data payload and there is substantially nobit-stuffing. Transfers happen under the worst of circumstances wheneach transfer involves maximum data payload and there is maximumbit-stuffing. Bit-stuffing occurs according to an embodiment of thepresent invention because the signals on the bus obey non-return-to-zero(NRZ) signaling. According to an embodiment of the present invention,bit-stuffing may increase the size of the maximum data payload by 16%.While in an embodiment, the best circumstances and the worstcircumstances have been defined in terms of bit-stuffing, it should beapparent that in alternative embodiments the circumstances can bedescribed in terms of other things that can expand or decrease the size(or time) of transfers, or generate delays. The generation of a budgetlist in accordance with the present invention will now he described.While one way of generating a budget list is described herein, it shouldbe apparent that the spirit and scope of the present inventionencompasses other possible ways. A budget list is a list of theallowable periodic transactions that can occur in a particular frametemplate. A frame is a continuously occurring period of time defined aspart of the bus specification that is sufficient to provide for one ormore transactions. In an embodiment, a 1 millisecond time period isdefined for a frame. A different budget list is constructed for eachframe that has a different set of allowable periodic transactions. Aframe template is a description of a particular periodically repeatingframe that provides for some maximum number of transactions, eachtransaction having some maximum data payload. A frame contains somenumber of transactions of some actual data payload, while a frametemplate describes a potential budgeted frame. Each budget list has anassociated best case information and an associated worst caseinformation. The best case information describes the situation in whicheach transaction in the frame template occurs under the bestcircumstances. In an alternative embodiment, transfers rather thantransactions are represented by the best case information and worst caseinformation.

FIG. 8 a illustrates a diagram for a best case frame template 800 inaccordance with the present invention. Block 805 represents the transferassociated with a first endpoint, where the transfer happens under thebest case. Block 810 represents the transfer associated with a secondendpoint, where the transfer happens under the best case. The remainingblocks 815-835 represent similar best case transfers for other endpointsconfigured by the system. The worst case information describes thesituation in which each transfer in the frame template occurs under theworst circumstances. FIG. 8 b illustrates a diagram for a worst caseframe template 850 in accordance with the present invention. Block 855represents the transfer associated with the first endpoint, where thetransfer happens under the worst case. Block 860 represents the transferassociated with the second endpoint, where the transfer happens underthe worst case. The remaining blocks 865-885 represent similar worstcase transfers for other endpoints configured by the system. It shouldbe apparent that the relative sizes of the blocks are only meant forpurposes of illustration.

The significance of best case frame template 800 and the worst baseframe template 850 will now be explained by examining blocks 805, 810,855 and 860. Blocks 805 and blocks 855 represent the best case and worstcase transfer, respectively, for the first endpoint. The earliest thatthe transfer for the first endpoint can finish is Ta. The latest thatthe transfer for the first endpoint can finish is Tb. The foregoingsuggests that the transfer associated with the second endpoint can beginas early as time Ta or begin as late as time Tb. Blocks 810 and 860represent the best case and worst case transfer times, respectively, forthe second endpoint. The earliest that the transfer for the secondendpoint can end is Tc. The latest that the transfer for the secondendpoint can finish is Td. The foregoing suggests that the informationnecessary for the second transaction must be available to the hub beforetime Ta and any result due to the first transfer is substantiallycertain to be available after time Tb. Furthermore, it should beapparent that scheduling of a transfer depends on the time required toperform each of the previous transfer requests. Additionally, it shouldbe apparent that the scheduling of the complete-split transaction willdepend on the time to perform the present transfer request, if any,associated with a start-split transaction.

Frame template 800 and frame template 850 are frame templates that arerepresentative of a frame between the hub and the agent (hub-agent orclassic frame). According to an embodiment of the present invention, thehost controller and the hub have another frame that is a fraction of thesize (microframe) of the classic frame. More specifically, in anembodiment, eight microframes are equivalent to a single classic frame.Since the host controller and the hub communicate using microframes andsince a transfer request before reaching the hub starts as a start splittransaction, it is useful to describe the time for performing the startsplit transaction in terms of microframes. Similarly it is useful todescribe the time for performing the complete split transaction in termsof microframes. With regards to the start time for a start splittransaction, according to an embodiment, a start split transactionshould occur one microframe before the microframe in which the transferbetween the hub and agent can occur according to the best case frame.With regards to the start time for a complete split transaction,according to an embodiment, a complete split transaction should occurone microframe after the microframe in which the transfer of theassociated start split transaction can finish according to the worstcase frame. According to an embodiment of the present invention, thehost controller uses the best case frame and the worst case frame foreach frame template to create a start-split vector and a complete-splitvector for each endpoint in each frame template. The start-split vectorcontains a value indicative of the microframe in which the start splittransaction for an endpoint should occur during a classic frame.According to an embodiment, start-split vector has values ranging from−1 to 6. The complete-split vector contains a value indicative of themicroframe in which the complete split transaction for an endpointshould occur during a classic frame. According to an embodiment, thecomplete-split vector has values ranging from 1 to 8.

When host controller 110 is tasked by host controller driver 105 toperform a transfer in a particular frame for a particular endpoint, hostcontroller 110 examines the best case vector that is associated with theendpoint and the frame template that corresponds to the particularframe. The value in the start-split vector for a given endpoint andframe template indicates the microframe at which a start splittransaction is to be performed. Similarly, host controller 110 examinesthe complete-split case vector to determine the microframe at which acomplete split transaction is to be performed.

FIG. 9 includes a sequence of hub-agent frames (or classic frames) 900and a host controller-hub frame 950. Frames 900 include frames 910-930.Frame 930 has a transfer 901 which is scheduled to start in microframe Band is scheduled to end in microframe B. In this illustration, transfer901 is an isochronous out transfer, but it should be apparent that othertransfers can be given a similar representation. Frame 950 includesframe 960 which has subframes 961 and 963. While transfer 901 isscheduled to occur in hub 120 in microframe B and end in microframe B(i.e., in the hub-agent time frame represented by frames 900), theassociated start-split transaction occurs in frame 961 and theassociated complete-split transaction occurs in frame 963. In the hostcontroller-hub time frame represented by frame 950, the associatedstart-split transaction occurs at time 951 a sometime in subframe 961and the associated complete-split transaction occurs at time 963sometime in subframe 963. Transfer 951 represents transfer 901 in thehost controller-hub time frame. Other transfers and transactions canalso occur in subframes 961 and 963. It should be apparent from FIG. 9that subframes 962-3 are available for other transfers and transactionswith other agents.

While the above description has generally been presented in the contextof periodic transactions, the invention is not limited to periodictransaction but also includes asynchronous transfers such as the bulkand control transfers described above. According to an embodiment of thepresent invention, ninety percent of a classic frame is set aside forperiodic transfers. Asynchronous transfers will occur at the earliesttime there is space available for them on the bus between the hub andthe agent. The hub will accept a transaction during a microframe when ithas space available to hold the start-split transaction until it can beissued on the classic bus (i.e., between the hub and an agent). Then thehub will issue the transaction during a classic frame when it has noother pending periodic transactions. Once the hub has accepted astart-split, the host controller will later issue a complete-splittransaction to retrieve the results from the hub for that transaction onthe classic bus. The host controller driver issues an asynchronoussplit-transaction to the hub whenever it has no other periodictransactions to issue during a microframe. The buffering provided by thehub for bulk/control transactions is distinct and separate from thebuffering provided by the hub for periodic transactions.

A method and apparatus for buffering periodic transfer requests andprocessing them in accordance with this invention will now be described.FIG. 1 e shows in greater detail hub 120 of FIG. 1 a. According to anembodiment, hub 120 of the present invention includes ahost-controller-huh (or high-speed hub) controller 180, a hub-agent (orlow-speed) hub controller 181, a memory 182, a control unit 183, a clock183 a, a repeater 184, a routing logic 185, and ports 185-189. Thecontroller 180 performs split transactions between hub 120 and hostcontroller 110. Whenever a start-split transaction is received bycontroller 180 from the host controller 110, controller 180 records thecurrent microframe number in memory 182 as a timestamp for thetransaction. An alternate embodiment would record the microframe numberas a timestamp in the state records to separate one group oftransactions received by the hub during one microframe from thosetransactions received during another microframe. Controller 181 performstransfers between the hub and one or more agents. The transfersperformed by controller 181 are received during start split transactionsperformed between controller 180 and host controller 110. Memory 182 iscoupled to both high-speed hub controller 180 as well as low-speed hubcontroller 181. Memory includes a pipeline with a plurality of stages.According to an embodiment, the pipeline has five stages. Each stage ofthe pipeline corresponds to a microframe as defined above. Each stage ofthe pipeline has a plurality of transaction states (or transactionrecords). According to an embodiment, a stage of the pipeline has 19 orfewer transaction states or records. Each stage stores recordsrepresentative of transfers to be performed. A record may have severalfields, such as device and endpoint address, direction of transfer,transfer type, status, and data reference. Status indicates whether thetransfer is not ready (prevent the low-speed hub controller fromperforming it), pending (waiting for execution) or ready (has beenperformed by the low-speed hub controller). The data reference is apointer to a starting address in memory for data received (i.e., intransfer) from the agent or data to be sent to the agent (e.g., outtransfer).

Hub 120 includes control unit 183 and clock 183 a coupled to controlunit 183. According to an embodiment, clock 183 a generates a microframeindication; i.e., that one-eighth of the time duration of the classicframe has elapsed. Control unit 183 is coupled to memory 182 andmonitors the records in the stages and prevents the performance of atransfer by the low-speed hub controller 181 if the time at which thetransfer is to begin execution at the low-speed hub controller 181 ispast the time the transfer was scheduled to be performed or to havecompleted performance. Specifically, according to an embodiment, arecord whose associated time stamp indicates that it was received bycontroller 180 from host controller 110 one microframe earlier, is setto a pending status to allow the low-speed hub controller 181 to issuethe transaction on the classic bus. A record whose associated timestampindicates that it was received more than three microframes before thecurrent microframe indication but has not yet been performed by lowspeed hub controller 181 is marked as old to prepare it for a subsequentcorresponding complete-split transaction by the high speed hubcontroller 180. A record whose associated timestamp indicates that itwas received more than four microframes before the current microframeindication but has not, yet been performed or is currently beingperformed by low-speed hub controller 181 is aborted and removed fromthe pipeline. Clock 183 a is also coupled to the low-speed hubcontroller 181. Controller 181 sequences in order through the periodictransaction records that are marked as pending. According to anembodiment, controller sequences 181 through the pending recordsreceived in the earliest microframe before proceeding to the recordsreceived in the next earliest received microframe and so on. Accordingto an embodiment, controller 181 proceeds to the next earliest receivedmicroframe when control unit 181 receives a microframe indication. Whencontrol unit 181 receives a microframe indication it changes the statusof the transfers in the next earliest received microframe from not readyto pending, and flushes out the transfers that are stale as describedabove. To perform a transfer controller 181 transfers data to routinglogic 185 which gives controller 181 access to one of the ports 186-189.Routing logic 185 gives an agent access to either repeater 184 orcontroller 181 to allow the transfer of data at a high data rate or alow data rate depending on the agent's configured data rate. Repeater184 is coupled to controller 180 and routing logic 185. Repeater 184repeats signals received from (or destined to) controller 180 anddestined to (or received from) a high speed agent coupled to one ofports 186-189.

While sequencing through the pending records received in the earliestmicroframe, controller 181 operates in accordance with the followingrules. First, after performing a transfer between controller 181 and theagent, controller 181 will retrieve the next transaction record andperform the next pending isochronous or interrupt transfer. Second, ifthere is no pending isochronous or interrupt transfers, controller 181will perform a pending bulk/control transfer. Third, if there is nopending bulk/control transfer, controller 181 waits for the high-speedhub controller 180 to indicate a pending bulk/control or isochronous orinterrupt transfer.

When the low-speed hub controller performs a transfer a result is storedin the memory 182. The result may be either a handshake or data receivedfrom the agent. Transfers which have been performed have their statuschanged by the low-speed hub controller 181 to ready and their recordsare made available to the high-speed hub controller 180. When a completesplit transaction is received by the high-speed hub controller 180, thehigh-speed hub controller 180 examines the record of the most recentlyperformed transfer with the same addressing information and sends aresponse back to the host controller 110 based on the result of the mostrecently performed transfer. If the complete split transaction is notinquiring about a transaction with the same addressing information, thehigh-speed hub controller 180 will respond back with a NYET.

Thus, a method and apparatus for scheduling transfers between a hostcontroller and a hub has been described. Additionally, a method andapparatus for buffering and performing transfers at a hub has beendescribed. Although the present invention has been described withreference to specific exemplary embodiments, it will be evident to oneof ordinary skill in the art that various modifications and changes maybe made to these embodiments without departing from the broader spiritand scope the invention as set forth in the claims. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

1. A video display device, comprising: a display; a port to connect to abus of a host device; and a bus controller to perform a splittransaction between a hub controller coupled to a host controller and tothe video display device, said hub controller to perform a splittransaction with the host controller to effect a transfer of databetween the host controller and the video display device; wherein thehub controller is to perform a first transaction of the splittransaction and a second transaction of the split transaction with thehost controller, said second transaction being different than said firsttransaction; wherein the hub controller is to perform an agenttransaction with the video display device based on the firsttransaction; wherein the second transaction is based on the agenttransaction.
 2. The video display device of claim 1, wherein the hostcontroller is operable to perform a third transaction with the videodisplay device based on the first transaction; and wherein the secondtransaction is based on the third transaction.
 3. The device of claim 2,wherein the first transaction and second transaction are performed at afirst communication speed.
 4. The device of claim 2, wherein the thirdtransaction is performed at a second communication speed.
 5. The deviceof claim 2, wherein the first transaction and second transaction areperformed in accordance with a first protocol.
 6. The device of claim 2,wherein the third transaction is performed in accordance with a secondprotocol.
 7. The device of claim 2, wherein the first transactionincludes a first packet sent from the host to the hub controller, thefirst packet including agent identification information, and the secondtransaction includes an acknowledgement received by the host controller.8. The device of claim 2, wherein the second transaction includes afirst packet sent from the host to the hub controller and a secondpacket sent from the hub controller to the host, and the second packetis based on data received from the host controller.